Elucidating Mechanisms of Palladium and Iridium Catalysis using Parahydrogen

This thesis aims to elucidate mechanisms of palladium and iridium catalysis using para-H2. In palladium catalysis, the debate on the nature of the active catalysts tends to focus on homogeneous palladium monomers or heterogeneous palladium nanoparticles; intermediate palladium clusters are repeatedly overlooked. The research herein aims to ascertain the role of such clusters in palladium catalysis. The conversion of palladium monomers to palladium clusters has been investigated. Reactive palladium monomers have been synthesised. Palladium-catalysed hydrogenation reactions were the focus and the suitability of these monomers for this study was verified by the formation of palladium nanoparticles on addition of hydrogen gas. Nanoparticles were characterised by transmission electron microscopy (TEM). A number of novel monohydride palladium species formed, and were characterised using liquid injection field desorption ionisation mass spectrometry (LIFDI-MS), and nuclear magnetic resonance (NMR) spectroscopy, including para-H2 induced polarisation (PHIP) which is used to enhance NMR signals.
Tri-palladium clusters were characterised by LIFDI-MS, advanced NMR spectroscopy, and single-crystal x-ray diffraction (XRD). These were isolated and found to be effective catalysts for hydrogenation reactions and mild Suzuki-Miyaura cross-couplings.
PHIP has also been applied in a non-hydrogenative manner using ‘signal amplification by reversible exchange’ (SABRE). This technique has been used to enhance the 1H NMR signals corresponding to pyridazine and phthalazine. The formation of SABRE-active iridium complexes with pyridazine and phthalazine and exchange of hydrogen and the N-heterocycle at iridium has been investigated. SABRE of the 1H NMR resonances was optimised. The potential to use drug derivatives of pyridazine and phthalazine as hyperpolarised contrast agents in magnetic resonance imaging (MRI) was verified by successfully hyperpolarising 5-aminophthalazine (an antifungal) and investigating SABRE in biologically-compatible solvents.